Service gateways are relay devices that connect networks having different protocols. A relay device connects, for example, a LAN (Local Area Network) and a WAN (Wide Area Network) and relays a packet received from a network on the LAN side or the WAN side to a network including a destination device. A LAN is a network provided in for example an office or the like of a company. The WAN side can serve as an Internet environment. A relay device is used for enabling devices in LANs to perform data communications via the Internet. In a network of a company, security such as a firewall or the like is set in order to prevent unauthorized accesses from outside. By setting a firewall, unauthorized accesses from outside can be prevented.
As a related technique, a technique is known that enhances security by making a relay device disconnect a line of the WAN side when an unauthorized access has been made from a network on the WAN side (see Patent Document 1 for example). Patent Document 1: Japanese Laid-open Patent Publication No. 11-225154